This invention relates generally to amplifiers and in particular to differential amplifiers.
Operational amplifiers and operational transconductance amplifiers (collectively referred to as amplifiers) are a basic building block of analog circuits, such as power amplifiers, feedback control systems, line drivers, buffers, filters, et cetera. Depending on the application, the construct of an amplifier will vary. For relatively imprecise and low-grade amplifiers, an amplifier may be constructed from a single transistor, where the transistor is operated based on its properties (e.g., transconductance characteristics for a field effect transistor (FET)).
The design of high performance amplifiers is an ever-increasing challenge, especially in light of the greater performance demands of precision analog circuitry. For example, amplifiers are now required to operate at lower supply voltages (e.g., down to 1.6 volts), but are still required to have a high output swing (2 volts peak-to-peak), increasing bandwidth, excellent linearity (up to 8 bits of linearity), and have good phase margin (90xc2x0+/xe2x88x923xc2x0). Some amplifiers have the further design challenge of having a tunable gain and bandwidth requirement.
Current amplifier designs have come close to fulfilling all of these design objectives but have had to compromise one or more design criteria in favor of another. For example, to achieve the high output swing requirement, amplifiers use a differential topology. As with any differential amplifier, the common mode of the output must be controlled to some level such that a maximum output swing may be obtained. Typically, the common mode is controlled using a common mode feedback circuit that controls the current through each leg of the differential amplifier based on a fixed reference common mode voltage. The common mode feedback circuit includes a current mirror and a fixed current source that provides the reference current for the current mirror. As such, the reference current sets the current that flows through each leg of the differential amplifier so that the output common mode voltage is equal to the reference common mode voltage.
While this circuit works well in many applications, the common mode of the differential amplifier will vary with temperature and component mismatches due to the fixed current source of the common mode feedback circuit. Further, if the amplifier is to be tunable (i.e., adjustable gain and bandwidth), the common mode may vary as the amplifier is tuned. As such, headroom and linearity of the amplifier are adversely affected.
Therefore, a need exists for an amplifier (an operational amplifier and/or an operational transconductance amplifier) that operates at low voltages, is tunable, is linear up to 8 bits, has good phase margin, and is insensitive to temperature, component mismatches and other common mode issues.